1. Field of Invention
The invention relates generally to the field of oil and gas production. More specifically, the present invention relates to a perforating system. Yet more specifically, the present invention relates to a modular initiator for use in a perforating gun system.
2. Description of Prior Art
Perforating systems are used for the purpose, among others, of making hydraulic communication passages, called perforations, in wellbores drilled through earth formations so that predetermined zones of the earth formations can be hydraulically connected to the wellbore. Perforations are needed because wellbores are typically completed by coaxially inserting a pipe or casing into the wellbore. The casing is retained in the wellbore by pumping cement into the annular space between the wellbore and the casing. The cemented casing is provided in the wellbore for the specific purpose of hydraulically isolating from each other the various earth formations penetrated by the wellbore.
Perforating systems typically comprise one or more perforating guns strung together, these strings of guns can sometimes surpass a thousand feet of perforating length. In FIG. 1 an example of a perforating system 4 is shown. For the sake of clarity, the system 4 depicted comprises a single perforating gun 6 instead of a multitude of guns. The gun 6 is shown disposed within a wellbore 1 on a wireline 5. The perforating system 4 as shown also includes a service truck 7 on the surface 9, where in addition to providing a raising and lowering means, the wireline 5 also provides communication and control connectivity between the truck 7 and the perforating gun 6. The wireline 5 is threaded through pulleys 3 supported above the wellbore 1. As is known, derricks, slips and other similar systems may be used in lieu of a surface truck for inserting and retrieving the perforating system into and from a wellbore. Moreover, perforating systems may also be disposed into a wellbore via tubing, drill pipe, slick line, coiled tubing, to mention a few.
Included with the perforating gun 6 are shaped charges 8 that typically include a housing, a liner, and a quantity of high explosive inserted between the liner and the housing. When the shaped charge high explosive is detonated, the force of the detonation collapses the liner and ejects it from one end of the charge 8 at very high velocity in a pattern called a “jet” 12. The jet 12 perforates the casing and the cement and creates a perforation 10 that extends into the surrounding formation 2.
The shaped charges 8 are typically connected to a detonating cord, which when detonated creates a compressive pressure wave along its length that initiates shaped charge detonation. An initiator 14 is typically used to set off detonation within the detonation cord. FIG. 2 provides a side cross sectional view of a typical initiator 14 having leads (16, 17) secured in an end cap 20 of the initiator 14 and connected on their lower terminal ends via a frangible bridge 18. The initiator 14 is typically controlled at surface where an electrical signal is sent via the wireline 5 to one of the leads (16, 17). In the example of FIG. 2 current from the electrical signal flows from lead 17 to lead 16 through the frangible bridge 18. The bridge 18 is made from a conductive material and includes generally a narrowed portion that heats and disintegrates under the applied current load. An amount of high explosive 22 is disposed in a housing 24 adjacent the frangible bridge 18 which is ignitable in response to the energy dissipated during the frangible bridge 18 disintegration. An end of a detonation cord 26 is positioned adjacent the lower end of the high explosive 22 and may be crimped 28 into place. Combustion of the high explosive 22 is readily transferred to the adjacent detonation cord 26 which detonates the cord 26 that in turn detonates the shaped charges 8.
Generally the initiators are connected to the perforating cords in the field just prior to use. Thus they are shipped to the field with the electrical portions and high explosive coupled together in a single unit. Because of the risks posed by the high explosives and the threat of a transient electrical signal, shipment and storage of the initiators is highly regulated, this is especially so when being shipped to foreign locations. Additional problems may be encountered in the field when connecting initiators to the detonation cord. Perforating guns when delivered to the field generally have the shaped charges and detonation cord installed; to facilitate initiator connection some extra length of detonation cord is provided within the gun. Connecting the initiator to the detonation cord involves retrieving the free end of the detonation and cutting it to a desired length then connecting, usually by crimping, the initiator to the detonation cord. These final steps can be problematic during inclement weather. Additionally, these final steps fully load a perforating gun and thus pose a threat to personnel in the vicinity. Accordingly benefits may be realized by reducing shipping and storage concerns, increasing technician safety, and minimizing the time required to finalize gun assembly in the field.